Calibration of a Terrestrial Laser Scanner - Institute of Geodesy and ...

2 1. Introductiondiodes are either PIN diodes or avalanche photo diodes (APD). Each LiDAR system deflects the laser beamto objects. Therefore, a deflection unit is required, which is able to alignthe laser beam towards the desireddirection. It is distinguished between rotating mirrors and oscillating mirrors. Dependingon the deflectionprinciple, the field of view is defined. Due to this deflection of the laser beam, laser scanning providesnearly continuous scanning of the environment instead of discrete surveying. The objects are described byan arbitrary point cloud, which means a laser scanner cannot survey specific points, e.g. edgesA LiDAR has the ability to acquireor corners.more than the 3D coordinates of the environment. Due to the detectionof the reflected laser beam by a diode, the amplitude or intensityof the laser beam contains additionalinformation and can be used as a fourth dimension. Thus, a LiDAR system acquires not only 3D coordinatesbut also an intensity value. Even though the intensity is dependent on several aspects, e.g. the rangeobject, surface properties of the object, and the angle of incidence, defined by the angleto thebetween incidentlaser beam and surface normal, the information is helpful in interpretations and classifications of laserscanning data.intensity images,Thus, the acquired point clouds of LiDAR systems can be interpreted as range images, asor as 3D data.Some terrestrial laser scanners are shown in Figure 1.1. Most of the manufacturers have a range of productsthat include several typesof laser scanners. Terrestriallaser scanners are a surveyinginstrument that isneither an automated total station nor a digital camera. Development of surveyinginstruments have envi¬sioned an 'all-in-one' instrument that includes the functionality of automated total station, digital camera,GPS, and level. Nowadays, the fusion between automated total station and GPS is not wishful thinking, itis reality. Since photogrammetry and laser scanning have complementary characteristics, e.g. high detailedintensity information and high detailed geometry information, a fusion isnot onlyconceivable but alsorequired [Przybilla, 2005]. Most of the laser scanners can be equipped with a digital camera.HDS 3000Leica GeosystemsGXTrimbleLMS-Z420iRieglIMAGER 5003Zoller+FröhlichLS 420 HE 20Faro TechnologiesCPW 8000CallidusFigure 1.1:Overview of terrestrial laser scanners.The operating range and the precision of LiDAR systems depend on the distance measurement technique.Figure 1.2 shows the typical distance technologies, cf. Section 2.1, and their range-precision-dependency.Triangulation is used for close ranges, the frequency-modulated continuous wave method (FMCW) is cat¬egorized between close ranges and medium ranges, the amplitude-modulatedcontinuous wave method(AMCW) operates in medium ranges and the direct time-of-flight method (TOF) is for medium ranges upto long ranges. Precision decreases as the range increases. Some typical applications are also given andcategorized with respect to the range in Figure 1.2.Terrestrial laser scanning is a promising technique and has the potential to be acceptedas additional sur¬veying technology. Recent developments regarding performance, range and accuracy have opened up newapplication areas for laser scanning. This is especially the case in the field of engineering geodesy. Sincethe first laser scanners showed several drawbacks and were not appropriate for geodesists, the next gener¬ations of terrestrial laser scanners have also taken into account the needs of geodesists, e.g. feasibility forlevelling, centering, and orienting. The data acquisition is also an integral part of terrestrial laser scanning.